Active pixel devices include a light sensing element, and internal image processing structure. Many active pixel sensors, such as that described in U.S. Pat. No. 5,471,515, the disclosure of which is incorporated by reference herein, have a buffer structure, e.g., a source follower, as part of each pixel.
Active pixel sensors can use photodiodes or photogates, or other light sensitive elements. In many photodiode type active pixels, the light sensitive element is directly connected to the active pixel amplifier and/or buffer.
A global shutter effect commands all the pixels to integrate for the same short absolute period of time. This effectively freezes the motion of objects. However, in a photodiode device, an external light blocking shutter has typically been used to stop the photodiode from accumulating photosignal.
FIG. 1 shows one approach. A sampling switch 102 and capacitor 104 are used. Sampling switch 102 is placed between the photodiode 100 and the capacitor 104. The capacitor charge node 106 is connected to the active pixel amplifier 110. The photodiode voltage is sampled by a closing switch 102 and allowing the charge from the photodiode to charge sampling capacitor 104. Switch 102 and capacitor 104 are covered by a metallic light shield to avoid pickup of undesirable light signal. This allows freezing the charge output at any given time.
However, the charge from the photodiode 100 is shared onto the capacitor 104 in the voltage domain. When the switch 102 is closed, the charge flows from the photodiode 102 in order to equalize the voltage between photodiode output and capacitor 106 node. Thus, the maximum signal-induced voltage swing of the capacitor 104 may be limited by the voltage of the photodiode. Since charge is shared, the voltage swing is in fact, always lower. Furthermore, turning off the sampling switch may itself inject charge to the sampling capacitor. This, in turn, adds offset and noise.